feat: setup-context7 skill + using-context7 prerequisites pointer

Split context7 concerns into two skills:
- using-context7 (policy, every-time): when to call, how to query well
- setup-context7 (infrastructure, one-time): install plugin, inject API
  key, clean manual MCP entries, with confirmation gates

setup-context7 reuses an existing CONTEXT7_API_KEY by searching, in order:
~/.claude/settings.json → ~/.claude.json (top-level + project-scoped) →
~/.claude/settings.local.json → env. Asks the user only if nothing found.
Cross-platform (paths identical on Win/Linux/macOS; only JSON validator
differs). Phase-2 confirmation gate before any write; per-edit JSON
validation; rollback procedure documented.

using-context7 gets a small Prerequisites section pointing at
setup-context7 when MCP tools are missing.

Bonus fix: scripts/build.sh PS multi-arg bug. Comma-joined -Names didn't
split into [string[]] when build.ps1 was invoked via -File. Now the bash
side loops one PS invocation per skill.

Wiki: extended .wiki/concepts/context7-setup.md with the design rationale
(why split policy from setup) — template for future "X plugin + how to
use X" pairs. index.md and log.md updated.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.wiki/log.md

@@ -19,3 +19,4 @@ Parseable: `grep "^## \[" .wiki/log.md | tail -20`.
 ## [2026-04-28] refactor | wiki-realignment — fixed project-bootstrap Step 3 to create Karpathy-canonical layout
 ## [2026-04-28] refactor | this repo's `.wiki/` migrated to canonical layout (SUMMARY.md→index.md, source/→concepts/, added log.md/overview.md/CLAUDE.md schema, raw/README.md)
 ## [2026-04-28] decision | context7-setup — switched to official plugin; --api-key injected into plugin's .mcp.json; three manual MCP entries removed
+## [2026-04-28] decision | setup-context7 skill — formalized the install/migrate algorithm; using-context7 gets a Prerequisites pointer; build.sh PS multi-arg bug fixed (loop instead of comma-joined -Names)